Palladium plating



Patented Dec. 21, 1948 PALLADIUM PLATING Edmund Merriman Wise,Westfield, N. J., and Raymond Francis Vines, Queens Village, N. Y.,assignors to The International Nickel Company, Inc., New York, N. Y., acorporation of Delaware No Drawing. Original application June 25, 1940,Serial No. 342,286, now Patent No. 2,335,821, dated November 30, 194.3.Divided and this application September 23, 1943, Serial No. 503,530.

in Canada May 20, 1940 4 Claims. (Cl.'20447) The present inventionrelates to the electrodeposition of palladium including theelectrorefining thereof and, more particularly, to the production ofsmooth, ductile, heavy electrodeposits of palladium.

Heretofore the electrodeposition of palladium has been limited to theproduction of thin electrodeposits of the order of about 0.00001 inch toabout (rarely) 0.001 inch thick. These thin electrodeposits are usefulfor decorative effects such as for protecting silver from tarnish.However, the prior art baths and processes were not satisfactory forproducing heavy, ductile platings.

The prior art baths may be divided into four main types depending uponthe type of palladium salt used. Pilets bath is an example of the firsttype in which the essential constituent is a complex ammonium palladiumsalt. Phosphates are usually present and it is assumed enter into thepalladium complex. The bath may also contain conducting salts, buffersand brighteners or grain refiners. Baths of this type, as, for example,those of Wise mentioned in U. S. Patent No. 1,991,995, have also beenemployed where extremely low metal ion concentrations are desired andhave contained ammonium plus cyanide compleXes. The baths of this typehave a low palladium ion concentration and are generally operated in theapproximately neutral or somewhat alkaline condition at relatively lowplating rates. Insoluble anodes of platinum, graphite and the like areusually employed since palladium does not dissolve to any appreciableextent in the electrolyte. Consequently, the bath must be replenished bythe addition of palladium salts to replace the palladium removed fromthe electrolyte by deposition at the cathode. These additions ofreplenishing salt lead to the accumulation of undesirable salts in thebath which eventually becomes so great that the bath must be discarded.Thin deposits from this type of bath are bright but heavier deposits aredark and brittle and are likely to peel from the basis metal.

A second type of palladium plating bath is typified by that described inU. S. Patents Nos. 1,779,436 and 1,779,457. These baths are quitesimilar to the Type 1 baths in all respects except that the essentialpalladium salt which is also used for replenishing the bath formsgaseous products and palladium on electrolysis. In this manner, thereplenishment of the palladium content of the electrolyte isaccomplished without causing the accumulation of other undesirablesalts. These baths are operated under conditions similar to those ofType 1 baths and the deposits are likewise bright when thin but dark andbrittle and likely to peel from the basis metal when heavy.

A third type likewise contains as its essential constituent a complexammonium palladium salt and is typified by Atkinsons bath described inBritish Patent No. 381,931. However, these baths differ in operationfrom those of Type 1 in that a porous cell surrounds the catholyte andpermits the migration of undesirable salts to the anolyte where they maybe removed. In this manner the accumulation of undesirable saltsintroduced in the replenishin solutions is avoided. Atkinson and Raperin discussing the results obtainable by the employment of baths of thethird type have stated that deposits up to about 0.00002 inch thick arebright but that thicker deposits are milky. While Atkinsons bath andprocess may be used for building up thick deposits of the order of about0.01 inch thick, nevertheless these relatively thick deposits have atendency to be brittle. (JnLEIectrodepositors Tech. 800., Volume 8,1933.)

A fourth type of bath has been found to give satisfactory thin palladiumdeposits. Typical of this type of bath employing complex alkali metalpalladium nitrite is that disclosed by Wise (U. S. 1,970,950). Thesebaths containing complex palladium nitrites may also contain chloridesor bromides as disclosed by Raper (U. S. 1,993,623). In the presence ofchlorides or bromides, palladium anodes dissolve quantitatively andconsequently the bath is self-replenishing. However, in the absence ofchlorides or bromides palladium salts must be added to the bath toreplace the palladium plated out and this likewise results inanaccumulation of undesirable salts in the bath. These bathslik'ewisemay be classified as low palladium ion'baths and are operated atapproximately neutral or slightly alkaline pH. According to Atkinson andRaper (loo. cit.) these baths are only useful for depositing a layer ofpalladium about 0.0001 inch or less in thickness since thicker depositsare very often unsatisfactory due to the fact that fine cracks appear inthe deposit.

From the foregoing brief rsum of the prior art processes it is apparentthat none of the prior art processes is suitable for depositing ductile,heavy electrodeposits of palladium. Furthermore, all the prior art bathsare of the low palladium ion concentration type resulting from the useof complex palladium compounds as the source of the palladium ion. Allthe prior art bathsof necessityare operated at practically neutral orslightly alkaline pH to prevent decomposition or precipitation of thecomplex palladium salt. As a result of the experience gained the art hastaught that these conditions were necessary for successful plating andfor this reason palladium plating baths have been restricted to those ofthe complex compound type. In fact, Atkinson and Raper (loc. cit.) statethe simple salts of the metal (palladium) as for example the chloride(PdClz) andthe sulfate (PdSOa) are not completely stable inaqueoussolution, and hydrolysis also occurs to some extent. Even in acidsolutions, such electrolytes are not suitable for electroplatingpurposes, since the deposits are dark and probably contain much basicmatter.

In contrast to this definite statement by those experts and in contrastto the teachings of the prior art, We have discovered that smoothductile heavy deposits of palladium may be obtained from a highpalladium ion concentration low pH bath.

It is an object of the present invention to provide a bath having highpalladium ion concentration and suitable for the electrodeposition ofpalladium.

It isanother object of-the present invention to provide a bath for theelectrodeposition of smooth, ductile, heavy deposits of palladium at arelatively rapid rate.

It is'a further object of the present invention to provide a bath forthe electrodeposition of palladium wherein the'palladium is present asachloride and the pH of the bath is low.

. The present invention likewise contemplates theinclusion in the-bathof conducting salts, .buiiering agents, grainrefiners and the like.

The .inventionslikewise contemplates the electrcwinning or refining ofpalladium from baths containing thepalladium as achloride and theproduction of-high purity 'palladium suitable for rolling .and annealingWithout'melting'. It is also within the contemplation of the presentinvention to employ the baths and-the process of the: present inventionin producing mirrors. as described in our copending application, U. 8;Serial No; 320,850, Patent No. 2,305,050, December 15, .1942, inproducing. or protecting of dental restorationsand particularly forprotecting'tooth pins and thelike, in producing thin walled tubing andin. surfacing electrical contacts. Other. objects and advantages will.become apparent from the following description.

Broadly speaking, the present bath employs.

increasethe tendency to hydrolyze and precipi tate a basic salt.Moderate additions of nitrates cause the deposit to assume a veryundesirable needle-like form when the bath is operated at hightemperatures. Fluoride additions, although. initially not exercising anynotable effect, later caused the deposit to become extremely brittle,probably due to reaction with the glass container employed. Depositablemetal ions other than palladium should of course beabsent unless analloyed deposit is desired. The presence of salts.

of tin, copper, lead, silver, aluminum and chrounder oxidizingconditions.

mium generally should be avoided as these el-' ments tend to producebrittle deposits. A simple palladium chloride-hydrochloric acid bath hasbeen found to give satisfactory results. However, the addition ofammonium chloride to the aforesaid simple bath appears to improve theductility of deposits obtained from certain lots of palladium anodeswhich otherwise produce rather less ductile plates.

We have found that the purity or quality of the palladium used inpreparing the bath and anodes affects the character of the deposit.Thus, we have found that ductile deposits are best produced from vacuummelted palladium heated to high temperatures such as about 1600" C. toabout 2000" C. or from palladium melted When normal commercial palladiumis used for preparing the bath and for anodes, thick electroplates canbe obtained but in some cases they have a tendency to be of moderatelylow ductility. Furthermore palladium melted under reducing conditions,especially reducing conditions produced by carbonaceous reducing agentssuch as city gas, generally yields plates with low ductility. Under suchconditions, that is, with normal commercial palladium or palladiummelted under reducing conditions, ammonium chloride additions to thebath improve the ductility of the plates produced from such palladium.

In contrast to the prior art baths our new palladium platin bath doesnot give entirely satisfactory results by direct plating on nickel,copper, iron, silver and the like. When it is necessary to plate suchless noble metals, a preliminary strike from a low metal ionconcentration bath of palladium, gold, platinum, rhodiumor othernoble'meta'l is preferably deposited first and then the final ductilethick plate of palladium isdeposited from our novel bath.

The palladium concentration of the novel bath may be-varied within widelimits, for example from about 10 grams per liter to about gramsperliter, i. e., a palladium ion concentration greater than about 10- thehigher concentrations permitting the use of higher current densities. Atthese higher concentrationsand higher current densities a deposit ofgiven thickness may besecured with considerably shorter plating times:However, with highly concentrated solutions, there is an increase in.the initial cost of the bath and the drag-out losses are higher.Therefore, concentrations of about 25 to about 50 grams of palladium perliter are preferable. The hydrochloric acid concentration may also bevaried within Wide limits as .baths containing as' little as 50 cc. andas much'as 700 cc. of concentrated hydrochloric acid per liter havegiven good below 1.0 but higher pI-Is may be used with baths of higheracid content. To the simple palladium chloride hydrochloric acid bath,ammonium chloride may be added in amounts of about 2 grams to about 50grams per liter. However, high concentrations of ammonium chloride areto be avoided in baths having high hydrochloric acid concentration as itis relatively insoluble in the presence of strong hydrochloric acid.

The character of the deposits obtained is dew pendent upon such factorsas palladium concentration, pH, temperature, current density andagitation and these factors are in turn interdependent as those skilledin the art know. Temperatures from 20 C. to 90 C, have been used withgood results but since the loss of hydrochloric acid at hightemperatures is objectionable, a temperature of about 50 C. ispreferred. In addition, current densities up to 50 amperes per squarefoot have given good results but a current density of about amperes persquare foot is generally preferred. With baths of high palladium contentoperated at high temperature and violently agitated much higher currentdensities can be employed, and under these circumstances the use ofinsoluble anodes may be preferred. In that case, the palladium'contentcan be maintained by dissolving palladium anodically in a separate cellequipped with a diaphragm to prevent the deposition of palladium on thecathode of said separate cell and causing the anodically dissolvedpalladium to flow into the plating cell. If the palladium anodesemployed be free from deleterious impurities no purification of theresulting solution is required, but if impure palladium be used it maybe necessary to purify the resulting solution prior to permitting it toflow into the plating cell. Alternatively palladium may be added as thehydrate or other suitable salt. It is likewise preferable to agitate theelectrolyte during plating and it has been found that air agitation issatisfactory. In order to give those skilled in the art a betterunderstanding of the present invention a few examples of baths andoperating conditions that have given satisfactory results are providedfor the purpose of illustration.

Plating baths suitable for depositing heavy,

ductile, relatively fine grained electrodeposits of palladium and theoperating conditions therefor are illustrated by the followingtabulation:

Constituent Range Preferred Palladium as PdClz... 25-175 gms. Liter 50gins/Liter.

Ammonium Ghloride 0-50 gms/Liter 20-50 gins/Liter.

pH (glass electrode) 0.5 to +2.0 1 lllll 00.5.

Temperature 25 C. to 35 C 50 0.

Current Density 5-50 amps/sq. ft 10 amps/sq. it.

Anodes Melted under oxidizing conditions. Agitation Air Lift Air Lift.

With low hydrochloric acid concentrations this pH is preferably keptbelow +1.0 but with higher hydrochloric acid concentrations higher pHvalues may be used. Under the preferred conditions, palladium may bedeposited at a rate of 0.00075 inch per hour or about 80 minutes per0.001 inch thickness.

While very satisfactory plates have been obtained from unbuifered baths,there are a number of buffers which may be incorporated in our newpalladium baths. Thus for example, boric acid, oxalic acid,trichloracetic acid, monochloracetic acid, phosphoric acid, citric acid,and acetic acid have been found suitable for buffering our new palladiumplating bath.

Example 1 324 grams of palladium are dissolved anodically inhydrochloric acid using a porous cell around the cathode to preventdeposition of the palladium. After solution is complete the palladiumchloride solution is boiled to remove excess HCl and chlorine and madeup to about 2 liters with distilled water. The pH of the solution isgenerally below 0. This bath when heated to 50 0., and operated using acurrent of about 1.1 amperes will deposit about 0.025 inch of palladiumon a, 3

6. inch by 1 inch specimen in about 16 /2 hours. The deposit is smooth,white, dense and fine grained. The edges are slightly heavier than thecenter of the plate and the composite sheet can be bent double with onlyslight edge cracking, that is to say, slight cracking at the edges wherethe deposit is substantially heavier than on the rest of the plate.

Example 2 92 grams of palladium are dissolved in aqua regia and thesolution so obtained evaporated to dryness. The residue is preferablytaken up with hydrochloric acid and water and evaporated .to dryness toexpel the nitric acid and other nitrogenous compounds. It has been foundthat three evaporations are generally suilicient to accomplish this. Thefinal residue is then taken up with hydrochloric acid and Water, 100grams of ammonium chloride added, the solution boiled and diluted to2000 cc. The palladium concentration is about 46 grams per liter and thehydrochloric acid concentration sufficient to give a pH of about 0.4. Athree inch by one inch sample plated at 0.5 ampere at 50 C. for about 18hours had a deposit about 0.013 inch thick equivalent to about 17.5grams of palladium. The deposit is smooth, white, and fine grained withonly a slight edge build-up and can be bent double without signs ofcracking. A bath such as this may be used for a long period of time andwire plated in such a bath is sufficiently ductile to be swaged cold asplated or after annealing.

E sample 3 grams of palladium are dissolved in aqua regia, the solutionevaporated to dryness and taken up with hydrochloric acid and water and.the nitric acid and nitrogenous bodies expelled as in Example 2 or inany similar manner. The final residue is dissolved in 1400 cc. ofconcentrated (37%) hydrochloric acid and diluted to about 2000 cc. withdistilled Water. The palladium concentration is about 40 grams per literand the pH about 2.1. When this bath is operated at 50 C. and a 3 inchby 1. inch sample plated With a current of about 0.5 ampere for about 19/2 hours about 20 grams of palladium are deposited providing a plateabout 0.015 inch in thickness with a deposit which is smooth, white,fine grained and very ductile.

The cathode current efficiencies in these new baths are very close toand the anodes corrode practically quantitatively. Therefore, the bathsmaintain their palladium concentration during use. Consequentlyadditions of costly palladium salts are not necessary, and theaccumulations of undesirable salts concomitant with the operation ofmany prior art baths is avoided.

In electrowinning palladium from impure palladium content anodes it maybe desirable to separate the anolyte and catholyte by a permeablediaphragm and to withdraw the anolyte (preferably from the bottom) andpurify it prior to returning it to the cathode compartment, so as topermit the production of a highly pure palladium deposit.

Although the present invention has been described in conjunction withcertain preferred embodiments thereof it is to be understood thatvariations and modifications may be made as those skilled in the artwill readily understand. Such variations and modifications are to beconsidered within the purview of the present specification and the scopeof the appended claims. Thus, calcium chloride and the like may be addedto the 7 bath to obtain grain refinement in a deposit; Buffering saltssuch as boric acid etc. may also be added. Oxalic acid, trichloraceticacid, monochloraceti-c acid, phosphoric acid, citric acid and aceticacid have been found suitable for buffering our new palladium bath.

This application is a division of Wise and Vines U. S, application Ser.No. 342,286, filed on June 25, 1940, now U. S. Patent 2,335,821, grantedNovember 30, 1943.

We. claim:

1. A process for electrodepositing thick, ductile coatings of palladiumwhich comprises establishing an aqueous bath containing about 10 toabout 175 grams per liter of palladium principally as palladium chlorideand about 50 cc. to 700 cc. per per liter of concentrated hydrochloricacid, said bath having: a pH not greater than about pH 2.0, immersing apalladium anode and a cathode in said bath, said anode consisting ofsubstantially pure palladium melted under non-reducing conditions, andpassing electric current through said bath whereby ductile deposits ofpalladium up to about 0.025 inch thick are obtained.

2. A process for electrodepositing thick, ductile coatings of palladiumwhich comprises establishing an aqueous acid bath having a pH notexceeding about pI-I 20, said bath containing about 25 to about 50 gramsper liter of palladium principally as palladium chloride and about 50cc. to 700 cc. of concentrated hydrochloric acid per liter, immersing apalladium anode and a cathode in said bath, said anode consisting ofsubstantially pure palladium melted under non-reducing conditions,

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,953,758 I-Iogaboom Apr. 3, 19341,993,623 Roper Mar. 5, 1935 2,335,821 Wise et a1 Nov. 30, 1943 OTHERREFERENCES Journal Electrodepositors Technical Society, vol. 8 (1933)10-5, page 8.

Electroanalytische Schnellmethoden, Fisher (1926) by Verlag vonFerdinand Enke, Stuttgard, page 165.

Helvetica Chimica Acta, vol. 4, pages 364-374 (1921).

Chemical Abstracts, vol. 15, 1921, page 3044.

Zeitschr'ift fur Elektrochemie, vol. 34 (1928), pages 237-240 and 744-752.

